Patients will be enrolled in clinical trials designed to treat patients with advanced Merkel cell carcinoma (stage 3b or 4). We seek to have the NIH Clinical Research Center serve as a study site in multi-institutional drug trials. Currently approval is being sought to participate in an open-label phase II study of EMD Serono's anti PD-L1 antibody (Avelumab) for the treatment of Merkel cell carcinoma. In addition to the samples collected during the trial for the central protocol, patients seen at the NIH will have additional tumor specimens and blood collected to evaluate anti-tumor immune responses during treatment. Trial participants will also have the option to enroll in a study on the microbiome in cancer immunotherapy.